Problem: Solve for $z$ : $24 = z + 15$
Explanation: Subtract $15$ from both sides: $ \begin{eqnarray} \\ \hphantom{24 {- 15}} &\hphantom{=}& \hphantom{z} \\ 24 &=& z + 15 \\ \\ {-15} && {-15} \\ \end{eqnarray} $ $ \begin{eqnarray} \\ 24 {- 15} &=& z \\ \end{eqnarray} $ Simplify. $ z = 9$